Concepts for detection of extraterrestrial life/Chapter 7
The J-Band Life Detector
This experiment is being studied for NASA by Dr. R. E. Kay and Dr. E. R. Walwick at the Philco Research Laboratories and is designed for use on Mars.
Because of the probable evolutionary history of the Martian environment, it is believed that Martian life will be based on similar chemical constituents and evolutionary principles as life on Earth. On Earth, life resides only in systems which are composed of molecular aggregates (macromolecules) known as proteins, nucleic acids and polysaccharides. Therefore, it is reasonable to assume that the detection on Mars of macromolecules having properties similar to proteins, nucleic acids or carbohydrates, will provide some support for the view that life exists on the planet. When certain dyes interact with macromolecules, color changes (metachromasia) occur which can serve to identify and detect biological materials. The present experiments have been concerned with the changes produced in the absorption spectrum of a dibenzothiacarbocyanine dye when it interacts with trace amounts of biological macromolecules. The spectral changes occurring when this dye reacts with biological macromolecules are unique in regard to the diversity of the changes that occur and the large amount of information which can be deduced.
In this case, the interaction of the dye with biological macromolecules always produces an increase in absorbance at new maxima. There are seven different regions of the spectrum in which absorption maximum are found. These are located at approximately 450, 480, 508, 535, 560, 575, and 650 mμ. The peak in the 650-mμ region is referred to as a J-band, being named after E. E. Jelly who described it in detail. This absorption band is particularly interesting because, of the macromolecules which have been tested, only those of biological origin interact with the dye to produce this absorption band. This is indeed fortunate, since the J-band has properties which make it especially useful in a detection scheme. It lies almost entirely outside the absorption region of the normal dye absorption spectrum, and the absorption coefficient is extremely high. Because of this, an increase in absorbance in the J-band region occurs in the presence of very low macromolecule concentrations and variations in the reference (dye band) are negligible. Thus, the experiment has been referred to as the “J-band life-detector.” This title is convenient because of its brevity, but it focuses attention on only one aspect of the method. The program is concerned not only with the J-band, but also with other alternations of the dye spectrum which result from the interaction of the dye with macromolecules.
The maxima which appear, and their exact wavelength, are functions of the macromolecule structure and the nature of its functional groups. Thus, for example, interaction of the dye with native deoxyribonucleic acid (DNA) produces a single peak at 575 mμ, whereas its interaction with denatured DNA causes a single peak at 540 mμ. On the other hand, proteins may produce multiple bands which occur in the 650-, 575- and 480-mμ regions of the spectrum. With proteins, a band can always be produced in the 650-mμ region of the spectrum; the exact wavelength of this band appears to be a function of the nature of the protein. Variables, such as change in acidity or alkalinity (pH) and temperature, cause changes in the absorption spectra of macromolecule-dye complexes which are related to the nature of the macromolecule. In general, the method is sensitive to 0.1 to 1 microgram of macromolecule per ml, and the absorbance of the associated complex is proportional to the concentration of the macromolecule.
The positions of the new absorption maxima appear to be a direct property of the spacing of the functional groups on the macromolecule, the rigidity of the macromolecule structure and the sequence of the anionic and cationic sites. The effects produced by changes in temperature and pH are apparently associated with modification in the folding and coiling of the macromolecule, the dye-macromolecule equilibrium and the ionizability of the functional side groups. Thus, by observing the spectral changes which occur when this dye interacts with a macromolecule and by appropriate manipulations of environmental variables, it is possible to detect trace amounts of macromolecules, distinguish between macromolecules which are difficult to differentiate by conventional methods and obtain information about the structure of macromolecules and estimate the macromolecule concentration.
It is expected that on Mars the experiment will be carried out in the following manner: The test capsule will acquire a soil sample, extract the macromolecules and mix them with the dye solution (sample solution and preparation). The absorbance of the dye-macromolecule mixture will then be determined.
In laboratory tests, a number of soil samples have been analyzed for macromolecules by the scheme indicated for the Mars experiment. In each case. changes in the absorption spectrum of the dye, indicative of the presence of macromolecules, were observed. This was true for soils which had a total organic carbon content as low as 0.2 percent. Concentrated extracts from some of the soils, which exhibited new absorption peaks in the 535- and 650-mμ regions of the spectrum, were analyzed for macromolecules by conventional laboratory methods. Macromolecules were isolated and amino acids and monosaccharides were obtained when the macromolecules were hydrolyzed, indicating that the macromolecules present in the soils were proteins and polysaccharides. This is in agreement with the results obtained by the dye test and strongly indicates that the method will be very useful for the detection of macromolecular species which are characteristic of all living material as we know it.
Figure 11 shows three absorption maxima. The first, at 505 mμ, is the region of normal dye absorption. The others, at 575 mμ and 648 mμ, are absorption bands due to the interaction of the dye with a 0.002-percent solution of oxidized ribonuclease.
Figure 11.—Three absorption maxima in the J-band region.